How to resume a render that got interrupted?
If you were rendering out frames as individual images, then you should be able to just set the "begin" frame to the last rendered frame, and go from there.

If you were rendering to a video file, then no... there is no way to you probably can't get all that work back. The file that got created (if it got created at all) is probably corrupt and unrecoverable. It's possible that the video file will still open in the VSE, and just have an unexpected end, or be shorter than it should be. I've never been able to, but IF you can open it, you can determine the end frame, render the remaining animation (to images), and stitch the two animations together after the fact. Furthermore, rendering the partial video file to individual frames might make that stitching process easier, but it will be critical to get the frame rate correct, or you'll end up with odd timing issues.

This is why it's a good idea to always render to individual frames, and then use the VSE to put those frames together into a video file. Once you have the frames rendered, rendering them into a video file is a very fast process.